Rising Greenhouse Gas Concentrations Driving Up Sea Surface Temperatures that Fuel More Intense Hurricanes

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In a Congressional briefing on 30 June 2010, hurricane expert Greg Holland of the National Center for Atmospheric Research (NCAR) said the potential for a disastrous 2010 hurricane season reflects not just natural variability but also climate change.  He explained that record high sea surface temperatures in the tropical Atlantic were one of the principal factors behind the dire forecast, and that rising atmospheric concentrations of greenhouse gases may account for roughly half of the anomalous warmth.  He warned that "we’re looking at potentially a doubling of major hurricanes in the next 20 to 30 years" as a result of global warming.  Holland, Director of NCAR's Earth System Laboratory, made his remarks as a member of a panel on Hurricanes and Oil Will Mix: Managing Risk Now

High Sea Surface Temperatures

A month before the panel's briefing, the National Oceanic and Atmospheric Administration (NOAA) issued on 27 May its 2010 Atlantic Hurricane Season Outlook. NOAA said there was a 70% chance that the Atlantic hurricane season would see 14-23 named storms, 8-14 hurricanes, and 3-7 major hurricanes. "Therefore, this season could see activity comparable to a number of extremely active seasons since 1995," NOAA said.  "If the 2010 activity reaches the upper end of our predicted ranges, it will be one of the most active seasons on record." 

Among the factors underlying its outlook, NOAA cited warm Atlantic Ocean surface waters, which in May were for the fourth month in a row at record high temperatures for the month:  "Sea surface temperatures [SSTs] are expected to remain above average where storms often develop and move across the Atlantic. Record warm temperatures – up to four degrees Fahrenheit above average – are now present in this region."

Sea Surface Temperatures at the Start of 2010 Hurricane Season.  Source: NASA Earth Observatory.

Above: Sea Surface Temperatures at the Start of 2010 Hurricane Season.  Source: NASA Earth Observatory.

"There has not been a seasonal forecast of 23 storms put out for this country before," said Holland.  Like forecasters from NOAA and elsewhere (see More pre-season predictions of a very active Atlantic hurricane season, at the WunderBlog for a summary of forecasts), Holland cited high SSTs as a principal factor underlying his assessment. The SSTs during the first month of the hurricane season did nothing to diminish concerns.  As the figure below indicates, high SSTs characterized the tropical Atlantic in June, with many areas again seeing record high temperatures for the month [Jeff Masters reports in a 14 July 2010 posting that "June 2010 is the fifth straight record warm month in the tropical Atlantic."]

"Map of monthly average Sea surface temperature (SST) anomalies (relative to 1961-1990) from HadSST2. White areas represent gridboxes containing no SST data. The black crosses and dashes indicate that a pixel is, respectively, the warmest or coldest example of that calendar month in the record."  Source: UK Met Office.

Comparisons to the 2005 Hurricane Season

Holland compared the 2010 hurricane season to the historic 2005 season that included the infamous Hurricane Katrina.  "If you just look at the patterns you’ll see that they’re actually quite similar," Holland said, referring to the two seasons.  He continued:

"We’ve got a very warm ocean out in the eastern Atlantic, not quite so warm ocean in the western Atlantic and into the eastern Pacific. The big difference is that the 2005, which was the previous record for the eastern Atlantic, is about a degree to a degree-and-a-half cooler than what we’re seeing at present. So we’ve got the same pattern, but it’s actually at unprecedented levels."

"So what’s happening?" asks Holland.  "Well it’s a combination of global warming and natural variability."  Holland displayed a graph showing a long term trend of rising SSTs coupled with considerable variability at different time scales.  The long term trend in SSTs and its impact on the intensity of hurricanes was noted in the report from NOAA's National Climatic Data Center, Hurricane Katrina, A Climatological Perspective, October 2005 (updated August 2006):

“A number of factors contributed to making Katrina a strong Category 5 hurricane (though weakening to Category 3 just prior to landfall). Sea surface temperatures (SST) in the Gulf of Mexico were one to two degrees Celsius above normal, and the warm temperatures extended to a considerable depth through the upper ocean layer. Also, Katrina crossed the “loop current” (belt of even warmer water), during which time explosive intensification occurred. The temperature of the ocean surface is a critical element in the formation and strength of hurricanes. As shown [below] ..., there has been an overall increasing trend in July-September Atlantic and Gulf of Mexico SSTs during the past 100 years marked by two distinct periods of increasing temperatures (1910-1945; 1976-present). This pattern is similar to that observed across global land and ocean surfaces.”

July – September SST Anomalies for 1880-2005, for the Gulf of Mexico (top graph) and Atlantic (bottom graph), (Smith and Reynolds, 2004 with updates).

July – September SST Anomalies for 1880-2005, for the Gulf of Mexico (top graph) and Atlantic (bottom graph).  Source: Hurricane Katrina, A Climatological Perspective, October 2005, Updated August 2006 , NOAA, 2006.

Global Warming May Account for Half of the Increase in Sea Surface Temperatures in Tropical Atlantic

When asked about the degree to which rising greenhouse gas (GHG) concentrations in the atmosphere were contributing to the trend of rising  sea surface temperatures in the tropical Atlantic Ocean, Holland said the temperatures could not be explained without accounting for rising GHG concentrations.  He said that while some researchers thought the rising GHG levels might account for 60-80% of the temperature anomaly, he estimated that about half was due to rising GHGs. 

This is consistent with research results published in Geophysical Research Letters on 29 April 2010.  In Is the basin-wide warming in the North Atlantic Ocean related to atmospheric carbon dioxide and global warming?, Chunzai Wang and Shenfu Dong of NOAA's Atlantic Oceanographic and Meteorological Laboratory, conclude that "both global warming and AMO [Atlantic multidecadal oscillation] variability make a contribution to the recent basin-wide warming in the North Atlantic and their relative contribution is approximately equal."

Long Term Outlook:

Holland summarized the long term outlook for hurricanes in a changing climate as follows:

  • "Annual frequency change is uncertain, some studies predict an increase, others a decrease."
  • "Very consistent predictions of intensity increasing by 5-10% in the mean."
  • "Potential 50‐100% increase in major hurricanes, consistent across all studies." "We’re looking at potentially a doubling of major hurricanes in the next 20 to 30 years," says Holland.
  • "Rainfall consistently projected to increase by about ~20%."  To underline the implications of this, Holland said that "the worst hurricane disaster we’ve had in the last 50 years in the North Atlantic was Mitch, which wasn’t even a hurricane.  It went ashore as a tropical storm in Central America. All of the damage, and the several thousand people who were killed was entirely due to rainfall."

"You really do need to take this in the context of a longer scale," Holland said, adding that "you need to start expecting more of this."  As if on cue, Hurricane Alex made landfall that evening (30 June) around 110 miles (180 km) south of Brownsville, Texas, as a Category 2 hurricane -- the strongest June hurricane since Hurricane Alma in 1966.  It was followed just over a week later by Tropical Depression Two with landfall at the southern tip of Texas. 

The Federal Emergency Management Agency reports today (12 July) that more than 20 inches of rain have fallen near the Texas-Mexico border from the two storms.  According to the Advanced Hydrologic Prediction Service, the Rio Grande at Loredo, Texas, crested at 42.45 feet on Thursday, 12.45 feet above major flood stage.  That would make it the worst flood in more than half a century at that location.  See Flooding Continues Along the Rio Grande (dtd 10 July) at the WunderBlog for video footage of the flooding on both sides of the border.

Additional Information

For additional discussion of NOAA's forecast for the Atlantic hurricane season, the role of elevated sea surface temperatures, and the connection to rising concentrations of greenhouse gases in the atmosphere, see our posting, NOAA Sees Potential for "Hyperactive" Hurricane Season; Record High Sea Surface Temperatures Among Contributing Factors (1 June 2010).

See also More pre-season predictions of a very active Atlantic hurricane season, posting (12 July 2010) by Jeff Masters on his WunderBlog.

The 30 June panel was moderated by Heidi Cullen, CEO and Director of Communications at Climate Central.  Other panelists included Rick Luettich, Professor & Director, Institute of Marine Sciences, University of North Carolina at Chapel Hill; and Rowan Douglas, who is the CEO of Global Analytics, Willis Re; and Chairman of the Willis Re Research NetworkSenator Mary Landrieu (Democrat, Louisiana) made additional remarks. 

The briefing was sponsored by the American Geophysical Union (AGU), the Congressional Hazards Caucus Alliance, the National Science Foundation (NSF), the Pew Center on Global Climate Change, the University Corporation for Atmospheric Research (UCAR) and the Weather Coalition.

A nearly complete transcript of the event is available at Gulf oil spill + Most active hurricane season ever = ?!? (1 July 2010), posted by Colin Schultz at  CMBR. 

All of the MS Powerpoint Slides [converted to PDF]. are available as a single PDF from the University Corporation for Atmospheric Research (UCAR).

Individual presentations:

(none of the audio files listed above include Holland's responses in the questions and answers at the end of the event)


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